412 General Notes, [April, 
the blastoderm opposite the embryo must grow in width more 
rapidly than, the embryonic half in which the embryonic shield is 
formed. 
These different modes of the growth in length of the bodies of 
embryos of different species of osseous fishes may easily be veri- 
fied by the observation of the progressive growth of the blasto- 
derm of the living ova, and go far towards reconciling the differ- 
ences of opinion which have been expressed by different observers 
as to the growth of the blastoderm over the yolk. It is at any 
rate evident that the manner in which this is accomplished in one 
form does not necessarily hold true of another. 
It is very significant that two large-yolked types, viz.: the Sau- 
ropsida and Elasmobranchii should both have the embryo dis- 
placed in position in reference to the margin of the blastoderm. 
In the latter, the first traces of the embryo have the normal mar- 
ginal position at the periphery of the blastoderm, but it is soon 
folded off, and before the yolk blastopore has closed, the latter 
finally closing a little way behind the stalk connecting the em- 
bryo and yolk sack. In the Sauropsida as held by Balfour, the 
primitive streak apparently represents the linear thickening be- 
tween the lower vitelline stalk and the point where the blasto- 
derm finally closed in the Elasmobranchii. 
___ It is thus made evident that, whereas the embryonic axis in 
Teleostei, Chondrostei, Ganoidei, Petromyzon and Amphibia ex- 
tends back to the point where the yolk blastopore closes, in Sau- 
ropsida and Elasmobranchii, the embryo is, partially folded off, 
and the tail begins to bud out before the vitellus is included by 
the blastoderm, and while the end of the axis of the embryo is 
still remote from the opening of the yolk blastopore. This con- 
trast between the two types, as will be evident to the thoughtful 
person, must be due to the great difference between the bulk of 
the yolks in the two cases. In the large-yolked forms if the em- 
bryonic axis were to continue to grow in length and extend quite 
to the point where the yolk blastopore closes, the body of the 
embryo would necessarily develop more somites than are present 
in the adult, so that growth in length of the embryonic axis 
ceases in the large-yolked forms far short of the point of closure 
of the blastoderm, covering perhaps only 30° of arc or less of the 
entire circumference of the vitelline globe. Such a small segment 
of the circumference of the vitelline sphere when contrasted with 
g0°-125°, and on up to 180° to 230°, embraced by the primary 
embryonic axis in Anamniate forms, seems inconsiderable, but is 
really relatively as extensive as in the latter. 
€ germinal disk of Sauropsida is relatively much larger than 
that of Teleostei, so that proportionally it probably does not 
spread over a much larger vitelline surface in the first case than 
_ In the last in order to include the vitellus, but as the blastoderm _ 
_ Spreads-in either case, it must be obvious to any one conversant 
